when saveorUpdate, an Error "a different object with the same identifier value was already associated with the session"

Question

Using Hibernate, I'm facing an exception saying a different object with the same identifier value was already associated with the session.

I don't know what's the cause of this exception and how to deal with it? If someone knows please teach me, thanks.

And next is a part of my code,

public void addRoleToUser(String userLogin, RoleBean roleToAdd)
  throws FunctionalException, TechnicalException {
    UserBean userBean = readInternal(userLogin);
    userBean.getRoles().add(roleToAdd);
    dao.save(userBean);  //dao is an instance of DAOCLASS
}

during the function of "dao.save"

try {
   session = sessionFactory.openSession();
   log.debug("session="+session.hashCode()+" save "+e);
   session.saveOrUpdate(e); //this line throws an exception
   session.flush();
}
catch (Exception e1) {
    //e1 now is "a different object with the same identifier value was already associated with the session"
}